Combined Stress

Question 1

On the column show, where will the stress value be greatest?

Answer 1

Point A (at face of column

Question 2

Pre-stressing a concrete beam primarily adds what type of stress to the section?

Answer 2

Compression

Question 3

Pre-stressed concrete members usually contain what type of reinforcement?

Answer 3

7-wire steel strand with fy = 250 or 270 ksi.

Question 4

Post tensioning can be used to reduce the deflection of a beam. (T/F)

Answer 4

True

Question 5

What is the most common prestressed material?

Answer 5

Concrete.

Question 6

In a fully prestressed concrete beam, how much of the cross sectional area can be considered effective?

Answer 6

The entire cross sectional area

Question 7

What is the main difference between pre-tensioned and post-tensioned
concrete?

Answer 7

These terms refer to the state of the concrete during the prestressing process. Pre-tensioning occurs before the concrete cures while post tensioning occurs after the concrete has solidified.

Question 8

Precast members are typically stressed using which method of tensioning?

Answer 8

Pre-tensioning.

Question 9

What type of stress is directly introduced by prestressing?

Answer 9

Axial force

Question 10

What is the main principle that supports the use of the interaction equation?

Answer 10

The principle of superposition

Question 11

What is meant by the term inverted king post? Or make a sketch of one.

Answer 11

A “King Post Truss” is a simple triangular truss with one vertical member at the center line. In an inverted king post, the lower diagonal chords are prestressed, placing the center strut in compression against the continuous top chord.

Question 12

For a member to be axially loaded, where must the load be placed?

Answer 12

At the centroid of the section

Question 13

For the member shown, what are the two forces that
need to be considered when calculating the
combined stress?

Answer 13

The moment Pe and the force P

Question 14

If the load is placed beyond the middle 1/3 of the section, what happens?

Answer 14

The face away from the load goes into tension

Question 15

Why should a column be located in the middle 1/3 of a spread footing?

Answer 15

To insure that the whole surface of the footing is bearing on the soil (in compression)

Question 16

Rafters are commonly analyzed using the moment based on the projected length. What force is neglected by this approach?

Answer 16

The axial force (which is relatively small)

Question 17

For the system shown below, which is greater, the axial force in the cable or the beam?

Answer 17

Cable
This is always true since
Ah=Bh, but the force in the
cable is Ah+Av

Question 18

In the system shown above, how would lowering point A effect the load in the
cable?

Answer 18

The force in the cable would increase

Question 19

In a P-Δ analysis of a compression member, where does the Δ come from?

Answer 19

deflection or initial crookedness

Question 20

Do post‐tensioned beams have internal stress before they are subjected to live
and dead loads?

Answer 20

yes

Question 21

Where are pre‐tensioned concrete beams commonly fabricated?

Answer 21

in a factory

Question 22

For a load‐balanced condition, the shape of the tendon must be the same shape as which diagram?

Answer 22

Moment

Question 23

What is the effect of attaching the tendon below the neutral axis of the top beam of an inverted king-post truss?

Answer 23

It produces a negative moment which offsets the positive moment from
gravity loads.

Question 24

What is the advantage of post-tension cables in the plane of a grid shell?

Answer 24

They increase the stiffness of the shell.

Question 25

In an eccentrically loaded column, when will tensile forces be developed?

Answer 25

When e is greater than or equal to b/6 (outside the middle 1/3)

Question 26

In a structural glass system such as that used in the entrance to the Renaissance Center in Detroit (the GM building) what is the primary force in the glass?

Answer 26

Tension, it is hung like a chain.